Among conventional power steering devices is one that reduces a steering force for a vehicle with use of an electric motor and an oil-hydraulic pump (see JP 2735743 B, for example).
In a schematic diagram of FIG. 7 is shown an example of a configuration of a power steering device that uses such an electric motor and such an oil-hydraulic pump. The power steering device has a vehicle ECU (Electronic Control Unit) 101, an inverter 102, a motor 103, an oil-hydraulic pump 104, a control valve 105, a power cylinder 106, and an oil reservoir tank 107. A piston 126 is driven by oil supplied from the oil-hydraulic pump 104 through the control valve 105, so as to produce a steering assist force.
The power steering device has an input shaft 112 having one end connected to a steering wheel 111 and the other end connected to one side of a torsion bar 113, and a worm shaft not shown and having one end connected to the other side of a torsion bar 113. The worm shaft is axially inserted into the piston 126 and is fitted in the piston 126 by a ball thread mechanism not shown. The piston 126 is connected to or formed integrally with an output shaft 114 and is movable in an axial direction of the piston 126. A motion of the steering wheel 111 in a steering direction is transmitted to the control valve 105 through the torsion bar 113, so that the control valve 105 is switched according to the steering direction of the steering wheel 111.
The output shaft 114 of the power steering device has a rack 115, with which a sector gear 116 meshes. Angles of tires 130, 130 on both sides are simultaneously changed by a link mechanism connected to a shaft of the sector gear 116.
The oil-hydraulic pump 104 and the control valve 105 of the power steering device are connected to each other through an oil supply passage 131 communicating with a pump port. One cylinder port of the control valve 105 is connected to one cylinder chamber of the power cylinder 106 through an oil passage 132, and the other cylinder port of the control valve 105 is connected to the other cylinder chamber of the power cylinder 106 through an oil passage 133. A discharge port of the control valve 105 is connected to a tank 107 through an oil passage 134.
The power steering device has a problem that communication between the pump port, both the cylinder ports, and the discharge port in a neutral position allows oil flow and causes additional consumption of electric power by the motor 103 when the assist does not work during straight-ahead driving, stoppage of the car, or the like. The power steering device has another problem that quick turn of wheel in steering passing through the neutral position causes oil from pump port side to flow to tank side and thus causes a steerer to feel a sense of sticking, because the oil is drawn into oil flow returning from the cylinder chamber to tank side on occasion of the passage through the neutral position.